1. Field of the Invention
The present invention relates to a lighting control apparatus for a photographing apparatus, and in particular to improvement of a signal wave which drives a lighting device.
2. Description of the Related Art
A lighting device which uses LEDs is proposed, in place of a lighting device which uses strobe radiation such as a xenon pipe which has been widely used with photographing apparatus, such as a camera. LEDs can be driven at a low voltage, and the circuit construction of the LED is simple.
However, the temperature of an LED goes up due to continuous radiation of the LED. The rise of the LED temperature causes a decrease in the quantity of light emitted by the LED. FIG. 1 shows the relation between the temperature rise and the quantity of light emitted by the LED, so that FIG. 1 shows that the quantity of light, in other words, brightness of the LED falls when the temperature of the LED rises.
Japanese unexamined patent publication (KOKAI) No. 2003-101836 discloses a lighting device for a photographing apparatus. The radiating of light from the LEDs is driven by rectangular wave signals (pulse signals), and a continuous shot operation can be carried out while the LEDs radiate light.
Because a turn-off period is provided in the pulse signal, the LED radiation caused by this pulse signal is not continuous, hence the temperature rise of the LED due to the heat that occurs with continuous radiation is reduced in comparison to the temperature produced by the LED radiation when the direct current signal is used to drive the LED.
FIG. 2 shows a graph where the horizontal axis represents time and the vertical axis represents temperature. The graph shows the difference in temperature rise of the LED due to the direct current signal ((1) in FIG. 2) and the temperature rise of the LED due to the pulse signal (the rectangular wave signal)((2) in FIG. 2).
When the pulse signal and the direct current signal are in the on state, the LED for lighting is illuminated.
In the case of the direct current signal, because a constant current flows during the period for which the LED should be illuminated (T0˜T5), the temperature of the LED continues to go up. When the direct current signal changes to the off state (T5), the temperature of the LED descends.
In the case of the pulse signal, the period for which the LED is not illuminated is the period which the signal is in the off state (T1˜T2, T3˜T4), so that the temperature of the LED descends in this period (T1˜T2, T3˜T4). Accordingly, during the illumination period of the LED, the temperature of the LED repeatedly rises and descends. As a result, the accumulation of heat is small in comparison with in the case where the direct current signal is used.
However, when the rectangular wave signals which are discussed in the above mentioned Japanese unexamined patent publication, are used, noise occurs in the power lines due to the rectangular wave signal switching between the on state and off state, noise occurs in the peripheral circuit due to the high-frequency component which is included in the rectangular wave signal, and a rush current results.